Exploring negative thermal expansion materials with bulk framework structures and their relevant scaling relationships through multi-step machine learning

Discovering new negative thermal expansion (NTE) materials is a great challenge in experiment. Meanwhile, the machine learning (ML) method can be another approach to explore NTE materials using the existing material databases. Herein, we adopt the multi-step ML method with efficient data augmentation and cross-validation to identify around 1000 materials, including oxides, fluorides, and cyanides, with bulk framework structures as new potential NTE candidate materials from ICSD and other databases. Their corresponding coefficients of negative thermal expansion (CNTE) and temperature ranges are also well predicted. Among them, about 57 materials are predicted to have an NTE probability of 100%. Some predicted NTE materials were tested by the first-principles calculations with quasi-harmonic approximation (QHA), which indicates that the ML results are in good agreement with the first principles calculation results. Based on the comprehensive analysis of the existing and predicted NTE materials, we established three universal relationships of CNTE with an average electronegativity, porosity, and temperature range. From these, we also identified some important critical values characterizing the NTE property, which can serve as an important criterion for designing new NTE materials.

Eu3+ doped CaWO4 nanophosphor for high sensitivity optical thermometry

Eu3+ doped calcium tungstate phosphors were obtained by using sodium citrate as chelating agent in hydrothermal process. The structure and morphology of the samples were indicated by XRD and the FE-SEM. The samples prepared by us are scheelite structure. In addition, the particle size of sample decreases with sodium citrate dosage increasing, and finally reaches the nanoscale. The average particle size is 90 nm. The temperature measurement properties of phosphors were tested. It can be seen from test results that the thermal quenching behavior of Eu3+ and WO42- luminescence has obvious difference. Hence, the FIR of Eu3+ and WO42- can be used to express temperature. The maximum relative sensitivity increases with the decrease of particle size and the maximum is 4.3% K-1 (303 K, 90 nm). Moreover, the color of sample luminescence altered continuously from blue to pink-red as the temperature increased. The luminous color of the sample can be used to roughly estimate the temperature. Therefore, the CaWO4: Eu3+ nanophosphor are promising materials for optical thermometry.

Broadband near-infrared luminescence properties of Sc2(MoO4)3:Cr3+ molybdates

The structural and spectroscopic properties of Sc₂(MoO₄)₃ molybdate containing various concentrations of Cr³⁺ ions were investigated in a temperature range of 80-300 K. The samples were prepared using hydrothermal as well as solid-state reaction methods. The influence of synthesis conditions and the molybdenum source on the structural properties was studied by X-ray diffraction (XRD), IR (infrared), and Raman methods. The optical properties of Sc₂(MoO₄)₃ samples doped with 0.1, 0.5, 1.0, and 2.0 % of Cr³⁺ ions were investigated. The broadband near-infrared (NIR) luminescence spectra generated from the ⁴T₂ and ²E levels of Cr³⁺ ions may be attractive for NIR light-emitting diode (LED) applications. Emission decay profiles and the crystal field parameters of Cr³⁺ ions are discussed. In particular, the mechanism of photoluminescence generation and the thermal quenching path are described in detail.

Tailoring Thermal Expansion of (LiFe)0.5xCu2-xP2O7 via Codoping LiFe Diatoms in Cu2P2O7 Oxide

Tailoring the thermal expansion coefficient of negative thermal expansion (NTE) materials to achieve near-zero thermal expansion has attracted great attention recently. Here, LiFe diatoms are adopted to substitute Cu in Cu₂P₂O₇ oxide to design Li-O-P and Fe-O-P linkages, with the stronger bond strength of Li-O and Fe-O compared to Cu-O and hence lowering the bond strength of P-O. With increasing the diatomic LiFe in (LiFe)_0.5xCu_2-xP₂O₇, new Raman bands corresponding to LiFeP₂O₇ appear and the NTE coefficient decreases gradually to near-zero thermal expansion at x = 1 (α_v = -0.90 × 10^-6 °C^-1, -100 to 55 °C). Comparing (LiFe)_0.5CuP₂O₇ with Cu₂P₂O₇ and LiFeP₂O₇, the average bond length of P-O increases while the bond angle of P-O-P decreases, and this is verified by some weakened vibrational energies of terminal PO₃ and P-O-P, resulting in the obvious red shift of Raman bands. Ceramic (LiFe)_0.5CuP₂O₇ presents a lower difference in grain size and a higher relative density than Cu₂P₂O₇ and LiFeP₂O₇.

Structure and Negative Thermal Expansion in Zr0.3Sc1.7Mo2.7V0.3O12

A₂M₃O₁₂-based materials have received considerable attention owing to their wide range of negative thermal expansion (NTE) and chemical flexibility toward novel materials design. However, the structure and NTE mechanism remain challenging. Here, Zr⁴⁺ and V⁵⁺ are used as a unit to compensatorily replace Sc³⁺ and Mo⁶⁺ in Sc₂Mo₃O₁₂ to tune its thermal expansion. Its crystal structure, phase transition, NTE property, and corresponding mechanisms are studied by high-resolution synchrotron X-ray diffraction, powder X-ray diffraction, ultralow-frequency Raman spectroscopy, and density functional theory calculations. The results show that Zr_0.3Sc_1.7Mo_2.7V_0.3O₁₂ adopts an orthorhombic (Pbcn) structure at room temperature, with V atoms occupying the position of Mo1 atoms and Zr atoms occupying the position of Sc atoms, and transforms to monoclinic (P2₁/a) structure at ∼133 K (45 K lower than that of Sc₂Mo₃O₁₂). It exhibits excellent NTE in a broader range. Most of the phonon modes below 350 cm^-1 have negative Grüneisen parameters, of which the lowest and next-lowest frequency (38.5 and 45.8 cm^-1) optical phonon modes arising from the translational vibrations of the Sc/Zr and Mo/V atoms in the plane of the nonlinear linkage Sc/Zr-O-Mo/V have the largest and next-largest negative Grüneisen parameters and positive total anharmonicity, and contribute most to the NTE.

Strong Negative Thermal Expansion in a Low-Cost and Facile Oxide of Cu2P2O7

Negative thermal expansion (NTE) behaviors have been observed in various types of compounds. The achievement in the merits of promising low-cost and facile NTE oxides remains challenging. In the present work, a simple and low-cost Cu₂P₂O₇ has been found to exhibit the strongest NTE among the oxides (α_V ∼ -27.69 × 10^-6 K^-1, 5-375 K). The complex NTE mechanism has been investigated by the combined methods of high-resolution synchrotron X-ray diffraction, neutron powder diffraction, X-ray pair distribution function, extended X-ray absorption fine structure spectroscopy, and density functional theory calculations. Interesting, the direct experimental evidence reveals that the coupling twist and rotation of PO₄ and CuO₅ polyhedra are the inherent factors for the NTE nature of Cu₂P₂O₇, which is triggered by the transverse vibrations of oxygen atoms. The present new NTE material of Cu₂P₂O₇ also has been verified for the practical application.

Thermal Evolution and Phase Transitions in Electrochemically Activated Sc2(MoO4)3

Sc₂(MoO₄)₃ shows negative thermal expansion (NTE) properties between -93 and +750 °C. Recently, electrochemical activation has been demonstrated to dramatically alter the phase evolution of structurally analogous Sc₂(WO₄)₃. Electrochemical activation involves placing the material of choice in an electrochemical cell with Li, Na or K counter electrodes and discharging (or reacting Li⁺, Na⁺ and K⁺) which is followed by extraction of the activated electrode and subsequent thermal treatment. Here such a process is applied to Sc₂(MoO₄)₃ and the results compared with the evolution of Sc₂(WO₄)₃. For 12.5% lithium discharged Sc₂(MoO₄)₃(12.5% of fully discharge capacity) the coefficient of thermal expansion (CTE) below 425 °C is -13.83(1) × 10^-6/°C which is larger than parent material, and a new Li_xMoO₂ phase forms at about 425 °C. The 25% lithium discharged Sc₂(MoO₄)₃ shows the formation of a new Li₂MoO₄ phase after discharging (electrochemical-based structural change) and on subsequent heat treatment the electrode mix transforms to Li₃ScMo₃O₁₂. Interestingly, a range of new phases at various temperatures in the sodium and potassium discharged samples appear during heat treatment. For example, Na_0.9Mo₂O₄ forms during thermal treatment of the 50% sodium discharged Sc₂(MoO₄)₃ while KMo₄O₆ and K₂MoO₄ form with thermal treatment of 100% potassium discharged Sc₂(MoO₄)₃. This work showcases the rich diversity in the phases that can be accessed during and post thermal treatment of Li, Na, and K discharged Sc₂(MoO₄)₃ electrodes.

Tailored phase transition temperature and negative thermal expansion of Sc-substituted Al2Mo3O12synthesized by a co-precipitation method

Sc³⁺substitution reduces theT_cof Al₂Mo₃O₁₂and makes it show stable negative thermal expansion in a wider temperature range.

Electrochemical performance and structure of Al2W3−xMoxO12

Al₂W_3−xMo_xO₁₂ shows an orthorhombic to monoclinic transition with increasing Mo concentration and ∼100 mA h g⁻¹ capacity at 100 cycles in Li-cells.

Negative Thermal Expansion of Ultrathin Metal Nanowires: A Computational Study

Most materials expand upon heating because the coefficient of thermal expansion (CTE), the fundamental property of materials characterizing the mechanical response of the materials to heating, is positive. There have been some reports of materials that exhibit negative thermal expansion (NTE), but most of these have been in complex alloys, where NTE originates from the transverse vibrations of the materials. Here, we show using molecular dynamics simulations that some single crystal monatomic FCC metal nanowires can exhibit NTE along the length direction due to a novel thermomechanical coupling. We develop an analytic model for the CTE in nanowires that is a function of the surface stress, elastic modulus, and nanowire size. The model suggests that the CTE of nanowires can be reduced due to elastic softening of the materials and also due to surface stress. For the nanowires, the model predicts that the CTE reduction can lead to NTE if the nanowire Young's modulus is sufficiently reduced while the nanowire surface stress remains sufficiently large, which is in excellent agreement with the molecular dynamics simulation results. Overall, we find a "smaller is smaller" trend for the CTE of nanowires, leading to this unexpected, surface-stress-driven mechanism for NTE in nanoscale materials.

Negative thermal expansion in 2H CuScO2 originating from the cooperation of transverse thermal vibrations of Cu and O atoms

Negative thermal expansion in 2H CuScO₂ originates from the cooperation of transverse thermal vibrations of Cu and O atoms.

Negative thermal expansion and associated anomalous physical properties: review of the lattice dynamics theoretical foundation

Negative thermal expansion (NTE) is the phenomenon in which materials shrink rather than expand on heating. Although NTE had been previously observed in a few simple materials at low temperature, it was the realisation in 1996 that some materials have NTE over very wide ranges of temperature that kick-started current interest in this phenomenon. Now, nearly two decades later, a number of families of ceramic NTE materials have been identified. Increasingly quantitative studies focus on the mechanism of NTE, through techniques such as high-pressure diffraction, local structure probes, inelastic neutron scattering and atomistic simulation. In this paper we review our understanding of vibrational mechanisms of NTE for a range of materials. We identify a number of different cases, some of which involve a small number of phonons that can be described as involving rotations of rigid polyhedral groups of atoms, others where there are large bands of phonons involved, and some where the transverse acoustic modes provide the main contribution to NTE. In a few cases the elasticity of NTE materials has been studied under pressure, identifying an elastic softening under pressure. We propose that this property, called pressure-induced softening, is closely linked to NTE, which we can demonstrate using a simple model to describe NTE materials. There has also been recent interest in the role of intrinsic anharmonic interactions on NTE, particularly guided by calculations of the potential energy wells for relevant phonons. We review these effects, and show how anhamonicity affects the response of the properties of NTE materials to pressure.

Low Temperature Synthesis and Characterization of AlScMo₃O12

Recent interest in low and negative thermal expansion materials has led to significant research on compounds that exhibit this property, much of which has targeted the A₂M₃O₁₂ family (A = trivalent cation, M = Mo, W). The expansion and phase transition behavior in this family can be tuned through the choice of the metals incorporated into the structure. An undesired phase transition to a monoclinic structure with large positive expansion can be suppressed in some solid solutions by substituting the A-site by a mixture of two cations. One such material, AlScMo₃O₁₂, was successfully synthesized using non-hydrolytic sol-gel chemistry. Depending on the reaction conditions, phase separation into Al₂Mo₃O₁₂ and Sc₂Mo₃O₁₂ or single-phase AlScMo₃O₁₂ could be obtained. Optimized conditions for the reproducible synthesis of stoichiometric, homogeneous AlScMo₃O₁₂ were established. High resolution synchrotron diffraction experiments were carried out to confirm whether samples were homogeneous and to estimate the Al:Sc ratio through Rietveld refinement and Vegard’s law. Single-phase samples were found to adopt the orthorhombic Sc₂W₃O₁₂ structure at 100 to 460 K. In contrast to all previously-reported A₂M₃O₁₂ compositions, AlScMo₃O₁₂ exhibited positive thermal expansion along all unit cell axes instead of contraction along one or two axes, with expansion coefficients (200–460 K) of α_a = 1.7 × 10⁻⁶ K⁻¹, α_b = 6.2 × 10⁻⁶ K⁻¹, α_c = 2.9 × 10⁻⁶ K⁻¹ and α_V = 10.8 × 10⁻⁶ K⁻¹, respectively.

Structure, phase transition, and controllable thermal expansion behaviors of Sc(2-x)Fe(x)Mo₃O₁₂

The crystal structures, phase transition, and thermal expansion behaviors of solid solutions of Sc(2-x)Fe(x)Mo3O12 (0 ≤ x ≤ 2) have been examined using X-ray diffraction (XRD), neutron powder diffraction (NPD), and differential scanning calorimetry (DSC). At room temperature, samples crystallize in a single orthorhombic structure for the compositions of x < 0.6 and monoclinic for x ≥ 0.6, respectively. DSC results indicate that the phase transition temperature from monoclinic to orthorhombic structure is enhanced by increasing the Fe(3+) content. High-temperature XRD and NPD results show that Sc(1.3)Fe(0.7)Mo3O12 exhibits near zero thermal expansion, and the volumetric coefficients of thermal expansion derived from XRD and NPD are 0.28 × 10(-6) °C(-1) (250-800 °C) and 0.65 × 10(-6) °C(-1) (227-427 °C), respectively. NPD results of Sc2Mo3O12 (x = 0) and Sc(1.3)Fe(0.7)Mo3O12 (x = 0.7) indicate that Fe substitution for Sc induces reduction of the mean Sc(Fe)-Mo nonbond distance and the different thermal variations of Sc(Fe)-O5-Mo2 and Sc(Fe)-O3-Mo2 bond angles. The correlation between the displacements of oxygen atoms and the variation of unit cell parameters was investigated in detail for Sc2Mo3O12.

Novel Materials through Non-Hydrolytic Sol-Gel Processing: Negative Thermal Expansion Oxides and Beyond

Low temperature methods have been applied to the synthesis of many advanced materials. Non-hydrolytic sol-gel (NHSG) processes offer an elegant route to stable and metastable phases at low temperatures. Excellent atomic level homogeneity gives access to polymorphs that are difficult or impossible to obtain by other methods. The NHSG approach is most commonly applied to the preparation of metal oxides, but can be easily extended to metal sulfides. Exploration of experimental variables allows control over product stoichiometry and crystal structure. This paper reviews the application of NHSG chemistry to the synthesis of negative thermal expansion oxides and selected metal sulfides.

Étude du système U-Sr-O par diffraction X à haute température

High-temperature X-ray diffraction (293 [Formula: see text] T(K) [Formula: see text] 1773) is used to investigate the reaction between strontium monoxide and uranium dioxide under controlled atmosphere (105 [Formula: see text] [Formula: see text] (Pa) [Formula: see text] 10-24), with lattice parameter measurements and composition estimates of the different uranates obtained. Thus, with a Sr/U = 1 sample, we successively observe the phases: (a) orthorhombic α-"SrUO4", whose reduction (3.67 [Formula: see text] O/U [Formula: see text] 3.62) is shown by a constant volume of the cell (V = 0.367 nm3) between 1173 et 1373 K; (b) rhombohedral β-"SrUO4", which shows a large composition variation between the metastable oxidized form (β-SrUO3.60) below 1108 K and the stable conjugate reduced form (β'-SrUO3.11) at whatever temperature; (c) "SrUO3" of constant composition (O/U [Formula: see text] 3) between 293 and 1533 K, then variable (O/U < 3) above 1533 K, with a probable second-order transformation (α-Pnma, β-Imma) for this distorted perovskite near 1073 K; (d) fluorite type U1-δSrδO2-δ solid solution for which a maximal account in SrO (δ [Formula: see text] 0.25) induces a 0.5 % thermal expansion parameter in comparison with UO2.00. A pseudo-binary "SrUO3"-"SrUO4" phase diagram is propounded. With a sample compound of Sr/U = 3, the monoclinic "Sr3UO6" perovskite is stable under [Formula: see text] [Formula: see text] 105 Pa up to 1373 K. On the other hand, in reducing atmosphere ([Formula: see text] [Formula: see text] 10-19 Pa), it becomes orthorhombic "Sr3UO5" with much greater lattice constants at every temperature.Key words: high-temperature X-ray diffraction, reactivity in metallic oxides, U-Sr-O system, nuclear fuels.